Although gas and vapor are two terms that are typically used interchangeably, the term “gas” is normally used for substances that exist completely as gases in room temperature. An example of a gas is oxygen. The term “vapor” is more commonly used for substances that generally exist as a liquid or solid at room temperature, although certainly capable of being present in a gaseous phase. Vapor pressure is a physical property of specific materials and is normally measured at a standard temperature of 77° Fahrenheit (25° Celsius). Chemical liquids or other substances that have a vapor pressure greater than the surrounding atmospheric pressure will evaporate into the atmosphere as a vapor and then diffuse outwards until an equilibrium pressure and uniform concentration is reached. Normally, the diffusing vapor (or gas) will reach equilibrium sooner if the volume of the space into which the vapor diffuses is confined or otherwise limited, as in a container, room or beneath a foundation slab. When the available volume into which gases or vapors can diffuse is limited, the resulting vapor pressure and gaseous concentration may reach equilibrium with the evaporating solid or liquid parent material (liquid being the more common of the two types of parent material) and no more material will evaporate. In this situation, the material will exist side-by-side in two different phases. If the available volume into which a gas is diffusing is essentially unlimited, such as the atmosphere itself as found in an uncovered outdoors location, the material will continue to evaporate and diffuse until it is all in the vapor state and the gaseous concentrations are so low as to be difficult to detect. Materials that have a tendency to evaporate at standard temperature and pressure are said to be volatile. If the materials are also organic compounds, they are called volatile organic compounds (“VOCs”).
A nonlimiting example of a VOC is perchloroethylene (also known by an assortment of common other names including perc, perchloroethene, tetrachloroethylene, tetrachloroethene, and a variety of trade names), the most widely used chemical in the dry cleaning industry. Perc has a vapor pressure greater than the normal atmospheric pressure of 14.7 p.s.i. at standard temperature and therefore behaves as a VOC and will evaporate. As a liquid, perc has a low interfacial tension and viscosity and readily penetrates into and through typical concrete slab foundations. Once perc has penetrated through a concrete slab into the subsurface beneath, perc begins or continues to evaporate. The accumulating perc vapors do not normally have sufficient pressure to migrate back upwards through the concrete slab (although human exposure to indoor VOC vapors moving upwards through expansion joints, cracks and other penetrations in slabs can become a problem). More often, the perc vapors effectively become trapped below the concrete slab in the pore spaces within the soil. These perc vapors move away from the source area, passing from pore space to pore space within the soil until the vapors become widely diffused in the subsurface beneath the concrete slab. The rate at which the VOC vapors accumulate and migrate depends on the amount of the liquid chemical that has been released, the organic content and the nature of the soil itself and the amount of fluid (normally water) that is also present in conjunction with the VOC or is already residing in the pore spaces.
It is important to detect VOC vapors as close to the source of the release as possible. A basic risk-based closure of facilities that have had a minimal impact or exposure to a VOC, e.g., perc, normally costs tens of thousands of dollars, even without the undertaking of any significant remediation or cleanup effort. Facilities that are significantly impacted, such as perhaps including ground water contamination, easily run into hundreds of thousands of dollars for remediation. This does not include the lost business opportunities, third party liability considerations and other miscellaneous damage claims.
There are a number of systems that monitor the VOC content of the air. Typically, these systems are expensive and difficult to maintain. If the subsurface has become impacted to the extent the VOC is in the aboveground air in a concentration that is capable of detection, the expense can already be overwhelming in terms of remediation and liability. Examples of this type of technology include that disclosed in U.S. Pat. No. 4,111,034, which issued on Sep. 5, 1978.
The present invention is directed to overcoming one or more of the problems set forth above.